“Therapeutic inertia” is a term coined over a decade ago to describe the 90% of clinicians who failed to intensify antihypertensive therapy in patients with high blood pressure (Antrade et al. Am J Manag Care 2004l;10(7 Pt 2):481-486). The concept was first applied to multiple sclerosis treatment decision-making three years ago and has become something of a cottage industry ever since, with about a dozen papers and abstracts to date on the topic.

The methodology has been to present a series of vignettes of MS cases. In most of the scenarios, treatment escalation would be appropriate because of ongoing disease activity. Disease activity was defined as 1 relapse and >4 new MRI lesions (or at least 1 gadolinium-enhancing lesion) (Saposnik et al. Front Neurol 2018;9:781). Failure to escalate in one or more scenarios is designated as “therapeutic inertia”. This would include clinicians who disagreed with the cut-off values for disease activity or would not necessarily switch therapies to a higher-efficacy disease-modifying therapy (fingolimod, natalizumab or alemtuzumab).

A study of 78 Canadian neurologists (mean age 38 years) reported that therapeutic inertia was present in 60% in at least one case scenario (Saposnik 2018). Perhaps a better indicator was that therapeutic inertia (failure to escalate) was present in 12% of participants’ responses.

However, other countries evidence greater inerte. A new study looked at therapeutic inertia among 226 neurologists in Canada versus Spain, Argentina and Chile; 195 physicians completed the study (Almusalam et al. JAMA Netw Open 2019;2:e197093). The overall rate of therapeutic inertia was 72.8%, which led to suboptimal treatment decisions in 20.4% of case scenarios. The 60% rate of therapeutic inertia in Canada was substantially lower compared to the other three countries (77.9%).

Factors that were associated with a lower rate of therapeutic inertia were seeing more MS patients per week, the number of years in practice and a lower aversion to ambiguity. It should be noted that the prevalence of MS is over two-fold higher in Canada (265/100,000) compared to Spain (125/100,000), Argentina (25.6/100,000) and Chile (11.7/100,000) (Rotstein et al. Neurology 2018;90:e1435-e1441. Fernandez et al. Mult Scler 2012;18:372-376. Correa et al. Mult Scler J Exp Transl Clin 2016;2:2055217316666407). It is unclear how the countries compare with respect to embracing ambiguity.